Add new API endpoint - get a namespace by ID

doc/api/namespaces.md

@@ -89,3 +89,35 @@ Example response:
   }
 ]
 ```
+
+## Get namespace by ID
+
+Get a namespace by ID.
+
+```
+GET /namespaces/:id
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id`      | integer | yes | ID of the namespace |
+
+Example request:
+
+```bash
+curl --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/namespaces/2
+```
+
+Example response:
+
+```json
+{
+  "id": 2,
+  "name": "group1",
+  "path": "group1",
+  "kind": "group",
+  "full_path": "group1",
+  "parent_id": "null",
+  "members_count_with_descendants": 2
+}
+```

lib/api/namespaces.rb

@@ -19,6 +19,30 @@ module API
         present paginate(namespaces), with: Entities::Namespace, current_user: current_user
       end
+
+      desc 'Get a namespace by ID' do
+        success Entities::Namespace
+      end
+      params do
+        requires :id, type: Integer, desc: "Namespace's ID"
+      end
+      get ':id' do
+        namespace = Namespace.find(params[:id])
+        authenticate_get_namespace!(namespace)
+
+        present namespace, with: Entities::Namespace, current_user: current_user
+      end
+    end
+
+    helpers do
+      def authenticate_get_namespace!(namespace)
+        return if current_user.admin?
+        forbidden!('No access granted') unless user_can_access_namespace?(namespace)
+      end
+
+      def user_can_access_namespace?(namespace)
+        namespace.has_owner?(current_user)
+      end
     end
   end
 end

spec/requests/api/namespaces_spec.rb

@@ -91,4 +91,73 @@ describe API::Namespaces do
       end
     end
   end
+
+  describe 'GET /namespaces/:id' do
+    let(:owned_group) { group1 }
+
+    shared_examples 'namespace reader' do
+      before do
+        owned_group.add_owner(request_actor)
+      end
+
+      context 'when namespace exists' do
+        it 'returns namespace details' do
+          get api("/namespaces/#{owned_group.id}", request_actor)
+
+          expect(response).to have_gitlab_http_status(200)
+
+          expect(json_response['id']).to eq(owned_group.id)
+          expect(json_response['name']).to eq(owned_group.name)
+        end
+      end
+
+      context "when namespace doesn't exist" do
+        it 'returns not-found' do
+          get api('/namespaces/9999', request_actor)
+
+          expect(response).to have_gitlab_http_status(404)
+        end
+      end
+    end
+
+    context 'when unauthenticated' do
+      it 'returns authentication error' do
+        get api("/namespaces/#{group1.id}")
+
+        expect(response).to have_gitlab_http_status(401)
+      end
+    end
+
+    context 'when authenticated as regular user' do
+      let(:request_actor) { user }
+
+      context 'when requested namespace is not owned by user' do
+        it 'returns authentication error' do
+          get api("/namespaces/#{group2.id}", request_actor)
+
+          expect(response).to have_gitlab_http_status(403)
+        end
+      end
+
+      context 'when requested namespace is owned by user' do
+        it_behaves_like 'namespace reader'
+      end
+    end
+
+    context 'when authenticated as admin' do
+      let(:request_actor) { admin }
+
+      context 'when requested namespace is not owned by user' do
+        it 'returns authentication error' do
+          get api("/namespaces/#{group2.id}", request_actor)
+
+          expect(response).to have_gitlab_http_status(200)
+        end
+      end
+
+      context 'when requested namespace is owned by user' do
+        it_behaves_like 'namespace reader'
+      end
+    end
+  end
 end